Hierarchical Design and Evaluation of Processes To Generate Waste

A methodology is described for the hierarchical design and evaluation of processes to make waste- recycled feeds that can be exchanged, thereby furthe...
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Ind. Eng. Chem. Res. 2004, 43, 2508-2515

Hierarchical Design and Evaluation of Processes To Generate Waste-Recycled Feeds Raymond L. Smith† National Risk Management Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268

A methodology is described for the hierarchical design and evaluation of processes to make wasterecycled feeds that can be exchanged, thereby furthering efforts aimed at industrial ecology. The methodology consists of nine hierarchy levels that include identifying, transporting, handling, sorting, converting, and purifying the waste-recycled feeds. These levels provide the designer with a systematic procedure for developing waste-to-feed processes. Evaluations at each level show whether a process for recycling a waste is economically and/or environmentally beneficial, or if costs or environmental impacts are actually greater than those saved from using a waste stream. The systematic procedure for designs and evaluations allows one to look for other alternatives immediately when costs or impacts are found to be too high. An example for styrene tar waste shows how the method can be used to design and evaluate waste-to-feed processes. In addition, any part of the process that is currently yet to be designed can use the intermediate results of economic and environmental impact values as targets for the remainder of the design. These targets let the designer know what monetary and impact values to remain below for the process to be economically and environmentally beneficial. In the future, this methodology can be used to analyze specific recycling processes of interest and evaluate their economic and environmental viability. Introduction The exchange of waste to be used as a recycled feed has long been encouraged by practitioners of industrial ecology. Industrial ecology is a field that includes a number of methodologies: life cycle assessment, design for the environment, material and energy flow analysis, industrial symbiosis, etc. These methodologies are similar in that they all take a systems view. To consider a product or process, such as that of recycling waste into feeds, from a systems view means to look beyond the immediate effects, to consider how flows indirectly affect the environment, and to consider these effects in total. In this work, aspects of exchanging waste, in particular chemical waste, will be reviewed and a methodology for the hierarchical design of processes for producing waste-recycled feeds will be proposed. In this paper we describe the pertinent aspects of industrial ecology, exchanging waste, waste exchanges, and eco-industrial parks, which provide background for the proposed methodology. The hierarchical methodology for evaluating the economic and environmental aspects of exchanging waste, including design of the waste-to-feed process, is described. Intermediate results of the methodology provide economic and environmental benefits that act as targets for designing the remainder of the process. Background Industrial Ecology. The idea that an industrial system can be viewed as a type of ecosystem is an element of the field of industrial ecology.1 Industrial ecology considers that both nominal ecosystems and † Tel.: (513) 569-7161. Fax: [email protected].

10.1021/ie030746g

(513) 569-7111. E-mail:

industrial systems have material, energy, and information flowssthat an industrial system depends on the biosphere, just as other ecosystems do. These ideas have emerged from various cultures, as described in a review including the history of the field of industrial ecology.1 The term industrial ecology, correctly used to define a field that uses a number of methods, is often confused with industrial metabolism. Ayres2 defines industrial metabolism as “the energy- and value-yielding process essential to economic development”. Therefore, industrial metabolism is the industrial economic system made up of the flows of energy and mass (including waste2). As such, industrial metabolism can be studied through energy and mass balances to analyze industrial flows. Proponents of industrial ecology suggest that the field goes beyond the analysis of energy and mass flows. Erkman1 suggests that, by first understanding the industrial system, we can then restructure it to make it function like a natural ecosystem. Part of the confusion on the use of the terms “industrial ecology” and “industrial metabolism” might stem from the fact that in Ayres’ 2 Industrial Metabolism he went beyond analyzing industrial flows, and suggested that an improvement to the current linear transformation of materials would be an efficient process cycle that uses wastes as feeds. Similar ideas regarding industrial systems as ecosystems were put forth at the same time by Frosch and Gallopoulos,3 who are credited with making the ideas of industrial ecology popular through their writing and positions at General Motors. These authors expressed the idea that the “industrial ecosystem” could be analogous to biological ecosystems. Among other ideas they suggested that industrial ecosystems need to have wastes tailored for reuse and products designed for the use of recycled feeds.

This article not subject to U.S. Copyright. Published 2004 by the American Chemical Society Published on Web 04/07/2004

Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2509

Another term common in industrial ecology literature is “industrial symbiosis”.4 The analogy here is to biological symbiosis, where each organism benefits, also known as mutualism. In industrial symbiosis, organizations transfer materials, energy, and/or information in a mutually beneficial exchange. Thus, industrial symbiosis,4 a well-functioning industrial ecosystem,3 and a cycling industrial metabolism2 are essentially the same. As such, cycling or use of waste described by these similar terms is part of the field of industrial ecology. Exchanging Waste. The industrial ecology perspective of considering an entire system, where industrial symbiosis leads to an overall advantage, does not mean that single exchanges of waste are easy and obviously beneficial. To turn waste “trash” into waste “scrap,” four aspects have to be considered to make the exchange feasible: technical, economic, regulatory, and institutional/marketing.5 These aspects will be discussed in more detail. The technical aspects of exchanging waste are essential in matching a waste stream with a possible feed stream. For a waste feed to replace a virgin feed, the physical and chemical properties, purity, and quantity of the materials need to match.5 The quantity of material may at first seem inconsequential, but for both sides of an exchange to find it worthwhile, the quantities of material available and desired need to be similar. Otherwise, either the generator will be left with too much waste or the user will obtain too little feed to make the effort worthwhile. The concentration of the desired material is also of interest,6 as lower concentration wastes are only desirable if their prices are relatively high. This concentration factor will have to be considered in processing costs described below for economic evaluations. Finally, purity can be a strong concern if an impurity could damage the user’s process or product.5 Users of waste materials have to be willing to accept this risk, or at least feel that the economics of exchange make the risk worthwhile. The economic aspects of exchanging waste can be the deciding factor as to whether any transfer occurs. The key is that economics drive each exchange: an overall benefit will not make a series of exchanges occur.7 The economics of each exchange have been presented5 as the total benefit of an exchange, B, described by B ) CD + CR - CT - CA, where CD is the cost of disposal saved, CR is the cost of raw materials saved, CT is the cost of transportation, and CA is the cost of administrative handling and processing. (According to the U.S. EPA,8 the cost of administrative handling is on the order of $10 per ton of waste.) This total benefit of exchange must be positive for any deal to be worthwhile. For individual benefits one can use a fraction of disposal costs and a fraction of raw material costs to define the savings for the generator and user, respectively.5 These individual benefits do not have to be considered if an exchange occurs with another subsidiary of a company; however, the success of such exchanges might not be based on efficiency of converting wastes or profit, but simply the fact that it does convert the company’s wastes.9 Regulations on waste can severely limit exchanges. There are three areas of potential liability: public, thirdparty, and contractual.5 Public liability occurs due to government enforcement of policies and statutes, and can lead to criminal penalties or injunctive (cease and desist) orders. Third-party liability arises when someone

not directly involved in the transfer of waste seeks compensatory damages under tort law. Finally, contractual liability comes into play if the terms and conditions of a contract are violated. One of the major fears of waste generators is that they will be held liable for the waste transferred to someone else even after transfer. This is a concern described by Martin et al.10 after company site visits. On the other hand, regulators fear that companies will attempt sham recycling, where one makes a show of reusing a waste to avoid treatment procedures and costs.7 Companies are required to show that waste reuse is legitimate.9 Another detriment to recycling waste is the Resource Conservation and Recovery Act (RCRA) regulations’ “derived-from” rule, which says that a solid waste generated from a listed hazardous waste remains a hazardous waste.10 More recently, EPA11 excluded wastes derived from hazardous wastes listed solely for the characteristics of ignitability, reactivity, and/or corrosivity if the derived-from waste no longer exhibits those characteristics. Other chemicals are included through the Comprehensive Environmental Response Compensation and Liability Act (CERCLA), which defines liability for cleanup costs associated with chemical releases, by an expansion of the list of hazardous chemicals beyond those subject to RCRA.8 Generators who exchange waste have to be willing to accept these risks, or at least feel that the economics of exchange make the risks worthwhile. Final concerns in the exchange of waste include aspects of the involved institutions and marketing. The confidence between institutions (including constant supply, anonymity, and maintaining secrecy of their products/processes) is very important.5 Companies do not want to worry about whether the exchange will happen, whether government agencies will obtain their data, or whether competitors will learn about their products/processes. In addition, the timing must be right for the generator and user to know about each other’s needs.5 This marketing of waste available/needed is not a trivial matter. Waste Exchanges. Unless the exchange of waste is done within a company or has been established over time, the transfer will occur through a waste exchange. There are two types of waste exchanges: information clearinghouses and materials exchanges. As suggested by their name, information clearinghouses create lists of information to connect waste generators and users. The information available normally shows what and how much is available or desired. Such information is gathered and distributed periodically by clearinghouses, which are normally not-forprofit organizations, although not government organizations since companies prefer not to share information on their streams with the government.12 However, the National Materials Exchange Network, a national computer-based listing service, is funded by the EPA.8 On the other hand, materials exchanges take possession of waste streams. These companies identify the potential of waste streams, buy or accept the waste, analyze the waste properties, process it, and then sell it.12 A materials exchange needs technically qualified employees (and many more employees than a clearinghouse), plus equipment and facilities to operate. Compared to clearinghouses, materials exchanges add more value to the waste they receive, and also receive a larger amount of money for their processing and acceptance of higher risk.

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Eco-industrial Parks. When enough exchanges of waste occur within an area, one can call the location an eco-industrial park, the idea being that collectively the companies can achieve greater economic and environmental benefits than each would alone.4 These collective benefits arise from individual agreements: the overall system of processes and exchanges is not planned; rather material exchanges occur one by one.7 Material exchanges occur in essentially five ways: in waste exchanges, where trades are on a case-by-case rather than a continuous basis; inside a large facility or company, where subsidiaries can act as separate entities; at a colocated eco-industrial park; in a prescribed area but not colocated; as virtual organizations over a larger region (sometimes called a virtual eco-industrial park).4 In an attempt to plan eco-industrial parks (contrary to the need for individual agreements7), a number of groups have developed software prototypes. It is likely that two software programs were developed simultaneously at the U.S. EPA and Bechtel. The U.S. EPA tools go by the names Facility Synergy Tool (FaST), the Designing Industrial Ecosystems Tool (DIET), and the Regulatory, Economic, and Logistics Tool (REaLiTy).13 FaST is a database of inputs and outputs from different facilities, which allows one to consider different flows in an eco-industrial park. DIET is a linear programming optimization tool that considers environmental, employment, and economic objectives. REaLiTy provides information on the regulations, economics, and logistics of reusing certain flows. These tools show some similarity to the Bechtel tool.14 Bechtel developed a program that uses a database to match feedstock requirements with byproduct and waste streams. Bechtel pointed out that when exchanges occur one by one that it is unlikely that new opportunities will be pursued, because the incentives to change are reduced. As a result, there is a need to picture the entire exchange system at the beginning, which is possible through the tool Bechtel developed. A third software tool that cites Bechtel’s program was developed at Yale University.15 This tool, called MatchMaker!, generates possible matches between facilities on the basis of facility flows or standard industrial classification (SIC) flows. These matches can help companies to identify possible exchanges or help regional planners in developing eco-industrial parks. Sixteen North American eco-industrial parks have been identified by the Smart Growth Network, which is part of the Sustainable Communities Network.16 Fifteen of these eco-industrial parks are in the United States. These eco-industrial parks are found throughout the U.S. in various levels of development. The most established eco-industrial park, and the most famous one, is located in Kalundborg, Denmark.17 The five companies and the municipality involved have reduced resource use and environmental impact while increasing profitability. The companies that exchange waste include a power station, a wallboard manufacturer, a pharmaceutical/biotechnology company, a soil remediation company, and a refinery.4 These exchanges are not static: the refinery has doubled its capacity, the power station has changed fuels, and the pharmaceutical plant has changed product lines and changed the volume of other lines.4 Hierarchical Chemical Process Design. To exchange wastes that need processing to become feeds, one needs to discover or design such processes. This work focuses on a systematic methodology for designing

processes to generate these waste-recycled feeds. The conceptual design of processes through a hierarchical method of synthesis and analysis steps has become a common method for designing chemical processes.18 By considering a panoramic view of a process first (expressed in economic terms as product revenues minus costs of raw materials) and continually adding detail in levels (for example, reaction systems, separation systems, and heat integration) during the design, one can evaluate the economics of processes at each level. Such analyses can stop when the economic potential for level i, EPi (which is the annualized profit of all levels up to and including i), becomes negative.18 Hierarchical process design has been used for minimizing waste and avoiding environmental impacts in chemical processes. Douglas19 expanded his earlier work to develop alternatives and heuristics for waste minimization. To progress from minimizing waste to reducing environmental impacts in hierarchical process design, some investigators have applied the Waste Reduction algorithm (see description below). Smith20 developed a spreadsheet method that provides simple and quick economic and environmental evaluations. Halim and Srinivasan21 have developed a software-based methodology that suggests and evaluates environmentally friendly design alternatives. Of course, besides hierarchical methods, much more work has been done in environmentally conscious process design.22 Recently, Shonnard and Hiew23 have developed their Environmental Fate and Risk Assessment Tool (EFRAT), which they applied to a computer-simulated process. Waste Reduction (WAR) Algorithm. This work uses the U.S. EPA’s WAR algorithm to evaluate potential environmental impacts (PEIs). First conceived as a method for evaluating a pollution balance,24 the WAR algorithm has developed into a method for evaluating balance equations around processes in terms of potential environmental impacts.25,26 The impact categories that the method considers include the human toxicity potential by ingestion (HTPI), human toxicity potential by exposure (HTPE) through dermal and inhalation routes, aquatic toxicity potential (ATP), terrestrial toxicity potential (TTP), photochemical oxidation (smog) potential (PCOP), acidification potential (AP), ozone depletion potential (ODP), and global warming potential (GWP). The method uses balance equations for impacts for a process using the following equation:

dIsys/dt ) I˙ in - I˙ out + I˙ gen

(1)

where I˙ k is the potential environmental impact per time. A key aspect of the method is the database of impact scores, ψij (for impact category i and chemical j), which are multiplied by mass flow rates, Mj, to obtain the I˙ k values:

I˙ k )

∑j Mjψij

(2)

These impact per time calculations refer to the I˙ in and I˙ out values, where the potential environmental impact is that which would occur if the stream, whether entering or exiting a process, were released to the environment. Normally, one assumes that the process is run at steady state so that Isys does not change with time. After calculating I˙ in and I˙ out, one can then use eq 1 to determine I˙ gen for the process. I˙ gen is useful for exploring the effect a process has on changing the

Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2511 Table 1. Levels of the Design and Evaluation Hierarchy for Producing Waste-Recycled Feeds (1) identify waste streams (2) identify waste components (3) determine savings in economics and environmental impacts of using wastes (4) determine economic costs and environmental impacts of transporting materials (5) determine economic costs and environmental impacts of administrative handling (6) determine economic costs and environmental impacts of sorting wastes (7) determine economic costs and environmental impacts of converting wastes (8) determine economic costs and environmental impacts of purifying feeds (9) adjust the economic costs and environmental impacts for residual wastes

potential environmental impacts (i.e., the amount the potential impacts are increased or decreased due to the process). Another common evaluation metric of processes is I˙ out, because it relates the potential impacts of only the (nonproduct) exiting streams to the environment. If one wants to aggregate various impacts, a weighting factor can be added to eq 2, so that the I˙ k values of eqs 1 and 2 refer to combined impacts rather than each category alone. In this work the potential environmental impacts for each category are weighted equally, not on the basis of value judgments, but for ease of reporting as a total value. Others using this method to make decisions are expected to apply their own values (or those of their organization) to weight the various categories or to consider each impact category separately. Design Hierarchy for Producing Waste-Recycled Feeds This work suggests a method for designing and evaluating processes that turn waste streams into feeds. The term used for these streams is “waste-recycled feeds”, signifying their origins as waste, the processing they go through as recycling, and their new state as feeds. In addition to the design and evaluation of processes, the method will also show how economic and environmental targets for designing such processes can be determined (as described below). The nine levels for evaluating and designing processes to create waste-recycled feeds are outlined in Table 1. The method presents a systematic way to determine how such processes should be designed and, perhaps more interestingly, whether they should be developed at all. In effect, if no process can be designed to economically or environmentally recycle a waste stream, then recycling is not beneficial according to the economic or environmental measure. This is a point that is sometimes lost, that recycling can have negative economic and/or environmental consequences. In this work the method for determining whether recycling is beneficial is described, whereas future work may focus on specific recycling issues of interest. The first two levels of the hierarchy for creating waste-recycled feeds are identifying waste streams and waste components. In identifying waste streams, one can consider unused output streams, periodic waste streams, and streams that are currently burned for fuel or other low-value uses. These streams can have higher economic values and lower environmental impacts when they are recycled. The components of the streams need

to be identified as individual species (or at least according to chemical class depending on the use), and their physical properties need to be determined as well as their purity and quantity. As discussed in the background section, any buyer or converter of waste needs to know the product with which they are working. As a first pass through this hierarchy, one might make assumptions in identifying these streams, determine the costs and potential environmental impacts through the levels of the hierarchy, and then return later for a more thorough identification. Thus, the levels of Table 1 can be viewed as an iterative procedure. The third level of the hierarchy is to determine savings related to using waste streams. In terms of economics, one would expect that disposal costs will be saved and that virgin feed costs will be reduced. Thus, we can define an economic potential (annualized savings) at level 3 of the hierarchy as

EP3 ) disposal savings + value of replaced feeds (3) where this analysis is assuming a single identity is gaining from both aspects. A more detailed analysis could be done which considers multiple agents benefiting from different aspects of this and the remaining levels of the hierarchy. Along with the economic savings there are benefits to the environment of not allowing the streams to become waste and not producing virgin feeds. In other words, a stream that was allowed to enter the environment is redirected, producing a benefit to the environment. This benefit will be represented as a negative potential environmental impact, calculated simply as

PEI3 ) impacts avoided in using waste + impacts avoided in replacing virgin feeds (4) These impact calculations could portion the impacts avoided to certain agents, but this analysis assumes that the result for the overall system is the important value. This third level of the hierarchy gives a positive economic potential and a negative potential environmental impact that will each deteriorate toward zero as one proceeds through the rest of the hierarchy. Levels 4-9 each add costs (i.e., decrease EP) and increase impacts (i.e., higher PEI). Therefore, a structure similar to Douglas’ original design procedure has been developed where additional synthesis and analysis can only make a process appear less attractive. As a result one can stop evaluating processes when the economics or environmental values become unfavorable. The evaluations are certainly unfavorable when the values cross zero (i.e., EP negative and PEI positive). Another interesting aspect of the level 3 evaluations is that they represent targets for designing a process. The level 3 EP and PEI values are the highest possible; these values can be used to target designs that complete the remaining hierarchy levels. One knows from the EP3 value how much can be spent on the process, transportation, and administration, and the PEI3 value sets an upper bound for releases that cause impacts. When the EP and PEI values are recalculated for other levels, the targets are adjusted. Thus, at each level one can see how much money and potential environmental impacts are available for the beneficial production and exchange of waste-recycled feeds.

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In determining the remaining costs and impacts after level 3, one can proceed in an order that simplifies the calculations. For instance, if database values for costs and potential environmental impacts of transporting materials are available, it is simple to apply these values first, before the sorting, converting, and purifying processes are designed. Others might be very familiar with the processes for sorting, converting, and purifying the waste into feeds and apply the costs and potential environmental impacts of (one or more of) these processes first. The following description will describe the remaining levels in the order shown in Table 1. Level 4 considers transportation, which can include transporting wastes and/or feeds from generators to brokers (i.e., materials exchanges), from brokers to users, or directly from generators to users. The economic potential will decrease according to

EP4 ) EP3 - costs of transporting materials (5) and the potential environmental impact will increase as

PEI4 ) PEI3 + impacts of transporting materials (6) Note that, in analyzing the environmental friendliness of most chemical processes, transportation costs and releases to the environment for transporting waste streams are not included. Usually the analysis boundaries of a process include only the costs and releases for the process, so to take this practice into account, one could subtract the original waste transportation costs and impacts from the magnitude of the terms applied to eqs 5 and 6. The next level of the hierarchy, level 5, is concerned with administrative handling costs and impacts. These may arise from labor, buildings, energy use, etc. that are associated with administration of the new process being developed. It is possible that these aspects will be similar to those already existing at the various process locations. If the new process for recycling the wastes into feeds adds relatively small marginal administrative costs and impacts, then they can be ignored. This would not be the case when substantial additional infrastructure is required to create a wasterecycled feed. The equations for calculating the costs and impacts at level 5 are of a form similar to those of level 4:

EP5 ) EP4 - costs of administrative handling

(7)

EP6 ) EP5 - costs of sorting wastes

(9)

PEI6 ) PEI5 + impacts of sorting wastes (10) Level 7 of the hierarchy is where sorted waste streams are converted into usable feeds. This conversion is a chemical process that changes the components of the waste stream. Of course, some recycling may not require chemical conversion. However, for those chemical processes that require sorting, conversion, and/or purification, one could see these levels of the hierarchy as a starting point for conventional chemical process design.18,27 In addition, some processes for creating wasterecycled feeds might combine some aspects of the sorting, converting, and purifying steps (for instance, by reactive distillation28). Thus, these levels of Table 1 could be combined, although here the equations are presented separately:

EP7 ) EP6 - costs of converting wastes

(11)

PEI7 ) PEI6 + impacts of converting wastes

(12)

The final processing step of the hierarchy is purification, level 8. As discussed in the background section, purification can be extremely important because lowconcentration materials have less value and impurities in feeds can cause significant problems in processes. The final purity necessary will depend on the use of the waste-recycled feed, the proportion to which it can be blended with virgin feed, the propensity for the impurity to accumulate in the process that will use the wasterecycled feed, and the damage the impurity may cause to the process and/or product. The costs and impacts of purification are described by the equations

EP8 ) EP7 - costs of purifying waste-recycled feeds (13) PEI8 ) PEI7 + impacts of purifying waste-recycled feeds (14) The last level of the hierarchy is where one adjusts the economic potential and potential environmental impacts for the residual wastes that remain after the processing steps. The processing steps of levels 6-8 of the hierarchy may leave residual wastes that have costs and releases to the environment associated with them. In addition, it was assumed in level 3 that all of the waste would save money in terms of virgin feeds, so the feed cost for the new residual waste has to be added back. These costs and potential environmental impacts are applied to the EP and PEI values:

PEI5 ) PEI4 + impacts of administrative handling (8)

EP9 ) EP8 - costs associated with residual wastes (15)

Operations that affect the properties of the original waste stream begin at level 6, where the waste is sorted to prepare it for further processing. Sorting can be thought of as a physical process that separates components of a waste stream. The amount of sorting, and the costs and impacts associated with it, can range dramatically from presorted streams that require no modifications to combined wastes that need intensive efforts. For chemical processes, one could expect sorting to be a separation unit. The evaluating equations appear in the form

PEI9 ) PEI8 + impacts associated with residual wastes (16) Once these costs and impacts have been applied, the calculations of economic potential and potential environmental impacts are complete (assuming that necessary iterations have been done and other aspects such as regulations, purity constraints, marketing, etc. have been taken into account). If the economic potential is still positive, then recycling the waste into feed is a positive economic transaction. If the potential environ-

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Figure 1. Flow sheet for styrene production. Table 2. Flow Rates (lb/yr) for the Design of a Process To Turn Styrene Tar Waste into Recycled Feeds component

tar waste

sorted waste

converted effluent

residual waste

benzene toluene styrene ethylbenzene xylenes diethylbenzene polymers total

787 622 2361 1426 1093 2977 5199 14465

0 0 2361 0 1093 2977 5199 11630

3291 709 0 3757 2198 1605 70 11630

0 0 0 0 0 1605 70 1675

mental impacts are negative, then recycling is beneficial to the environment. Example Design and Evaluation for Producing Waste-Recycled Feeds An example system stemming from styrene manufacturing is considered to design a process for producing waste-recycled feeds. Styrene production can be thought of as starting with toluene hydrodealkylation to benzene, followed by benzene alkylation into ethylbenzene, and finally ethylbenzene dehydrogenation into styrene. A flow sheet representing these processes is shown in Figure 1. From the figure, a number of waste streams and components have been identified. The waste stream focused on here will be the styrene “tar” which comes from the bottoms of the styrene-refining column. The components of this tar are assumed to be benzene, ethylbenzene (EB), diethylbenzene (DEB), styrene, toluene, xylenes, and polymers (components and amounts identified using refs 29 and 30 with ref 31 used for identifying DEB). To complete levels 1 and 2 of the evaluation and design hierarchy, identifying streams and components, the amounts of each component are shown in Table 2 under the column “tar waste”. The amounts in the tar waste were determined using a patent29 plus a TRI Explorer facility report30 (using styrene as the basis for weight ratios of other components in the patent). The level 3 target values for economic potential and potential environmental impact were calculated as described above. The disposal savings were calculated as $3616 at $0.25 per pound of waste.32 The value of benzene feed saved was approximated assuming that all of the tar waste would be recycled at the cost of benzene, $0.18 per pound. Thus, EP3 ) $3616 + $2604 ) $6220. (The economic potentials and potential environmental impacts for this example are presented in

Table 3. Economic Potential Savings and Potential Environmental Impacts Avoided for a Process for Recycling Styrene Tar Waste into Feeds hierarchy level

EP ($)

PEI

hierarchy level

EP ($)

PEI

3-5 6

6220 5391

-4586 -4577

7, 8 9

4456 3736

-4560 -3290

Table 3.) The potential environmental impact, I˙ out, was calculated by multiplying the flow rates of the tar waste by their respective WAR scores (summed over equally weighted impact categories) with the polymer assumed to be an equivalent weight of styrene. No account was made for the impacts avoided by not using virgin feeds, with the resulting value being PEI3 ) -4586. These EP3 and PEI3 values are now targets for designing a process (including transportation and administration) that has lower economic costs and produces less potential environmental impacts than the original tar waste stream. Note that it has been assumed that the tar waste is released into the environment. While not literally expected, this is the best estimate currently available for the emissions from end-of-pipe treatment and residual waste that occur in the actual process. The transportation and administration costs and impacts are normally determined in levels 4 and 5. However, for this process it will be assumed that the recycling process occurs on site and the waste-recycled feeds are returned to the styrene-manufacturing process of Figure 1. Therefore, the transporting and administrative handling costs and impacts will be considered negligible in this example (i.e., EP5 ) EP3 and PEI5 ) PEI3). The process for sorting the tar waste is designed and evaluated at level 6. There may be many processes that could sort the tar waste. In this example an indirect heating process is used followed by a flash vessel.33 It is assumed that a clean split of benzene, toluene, and ethylbenzene can be evaporated from the rest of the tar waste. Table 2 shows the amounts of each component in the column “sorted waste”, with the differences from the tar waste having been evaporated. As shown in Figure 1 benzene, toluene, and ethylbenzene are feeds along the process chain for manufacturing styrene, so it will be assumed that they can be returned to the process (with any further purification occurring in the styrene process chain). Costs of a boiler and flash vessel have been determined using engineering judgment and equipment cost graphs,34 dividing the annual cost by 4 to account for this process being used only one-fourth

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of the year for tar waste recycling. This accounting assumes that the styrene tar waste is a periodic stream and that other periodic waste streams can use the process equipment during the remainder of the year. Costs associated with heating (the tar waste stream and evaporating the three sorted components) using natural gas were also incorporated, but on a relative basis these heating costs are very small. The resulting calculation is EP6 ) EP5 - $829 ) $5391. Calculations were also done for potential environmental impacts to account for burning natural gas35 and fugitive emissions. It was assumed that 0.1% of each stream escaped as a fugitive emission. The resulting change in impacts is small, PEI6 ) PEI5 + 9 ) -4577. These EP6 and PEI6 values represent new targets for the rest of the recycling process. Level 7 of the hierarchy is where sorted wastes are converted into feeds. In this example the design for converting the sorted wastes has been taken from patents29,36 (where the sorted waste and converted effluent distributions used in Table 2 can be found). The reactor for this conversion process is a pipe, assumed to be heated with the same boiler as used for sorting. This converted effluent shown in Table 2 is then separated in a condenser so that diethylbenzene and polymers remain as residual waste (see Table 2). For these cost calculations34 both the reactor and condenser are estimated to be used one-fourth of the year for this process, under the assumptions described above. Again, heating costs for raising the temperature and vaporizing the sorted waste are very small. The new costs give EP7 ) EP6 - $935 ) $4456. The potential environmental impact calculations are similar to those for sorting, with fugitive emissions being larger than natural gas burning impacts, although both are relatively small. As a result, PEI7 ) PEI6 + 17 ) -4560. Again, these new EP and PEI values are targets for designing the rest of the process levels. For purification it is assumed that the process chain for manufacturing styrene can handle the wasterecycled feeds of benzene, toluene, ethylbenzene, and xylene (represented by the differences between the “converted effluent” and “residual waste” columns of Table 2). Thus, the costs and potential environmental impacts of purification are assumed to be negligible (i.e., EP8 ) EP7 and PEI8 ) PEI7). Finally, the residual wastes have costs and potential environmental impacts that affect the EP and PEI values. The first cost is that of adding back the virgin benzene feed that was assumed to be replaced by all of the tar waste in level 3. In this example the feed value of the residual waste not recycled is $301. Also, the residual waste must be treated, which costs $419 according to the assumed $0.25 per pound price. Therefore, the final economic potential value is EP9 ) EP8 $720 ) $3736. This is a positive value, so recycling the tar waste into feeds is economically viable under the assumptions presented. The potential environmental impacts of the residual waste are substantial compared to those of other levels, PEI9 ) PEI8 + 1270 ) -3290. Since the PEI9 value is negative, there are benefits to the environment of recycling the tar waste into feeds. Future Work The styrene tar example provides an initial description of a process to generate waste-recycled feeds. However, more interesting work is expected to follow

in the form of examples where independent agents trade waste and recycled feed streams. This extension of the work will provide real world examples wherein each individual needs to win (i.e., the process of generating waste-recycled feeds must be a win-win activity). In addition, waste streams and recycling processes of interest could be analyzed using this method to inform decision makers. This work can also be considered a branching point for other types of process design and environmental analysis. In the hierarchy of levels, methods for determining the optimum designs for the sorting, converting, and purifying processes were not addressed. These designs could be developed using a number of methods, including hierarchical process design,18 numerical optimization,27 and pinch technology (i.e., mass exchange networks37), with supporting models and simulations. Also, in looking beyond the evaluations presented here, it would be interesting to extrapolate the analysis to include quantifiable sustainability metrics38 and/or a broader supply chain view through techniques such as life cycle assessment.39 Conclusions A methodology has been described to design and evaluate processes for the exchange of waste. The exchange of waste has been described in terms of industrial ecology, where eco-industrial parks or single waste transactions develop through mutually economic exchanges. The methodology uses a hierarchy of design and evaluation levels to determine the economic and environmental benefits of processes to produce wasterecycled feeds, with the understanding that economic gains allow companies to exchange waste for the benefit of the environment. The hierarchy of the methodology is divided into nine levels that include identifying, transporting, handling, sorting, converting, and purifying the waste-recycled feeds. The production of wasterecycled feeds from styrene tar waste was examined as an example to show how the method can be used to design and evaluate waste-to-feed processes. In addition to the design and evaluation of waste-to-feed processes, the methodology provides intermediate results as targets for designing the remainder of the process. Designs that meet these economic and environmental targets provide benefits that can foster the exchange of waste. Literature Cited (1) Erkman, S. Industrial Ecology: An Historical View. J. Clean Prod. 1997, 5 (1, 2), 1. (2) Ayres, R. U. Industrial Metabolism. In Technology and Environment; Ausubel, J. H., Sladovich, H. E., Eds.; National Academy Press: Washington, DC, 1989. (3) Frosch, R. A.; Gallopoulos, N. E. Strategies for Manufacturing. Sci. Am. 1989, 261 (3), 144. (4) Chertow, M. R. Industrial Symbiosis: Literature and Taxonomy. Annu. Rev. Energy Environ. 2000, 25, 313. (5) Terry, R. C.; Berkowitz, J. B.; Mohr, C. M.; Tratnyek, J. P.; Funkhouser, J. T.; Shick, B. C.; Somogyi, A. C. Waste Clearinghouses and Exchanges: New Ways for Identifying and Transferring Reusable Industrial Process Wastes; Arthur D. Little: Cambridge, MA, 1976. (6) Allen, D. T.; Rosselot, K. S. Pollution Prevention for Chemical Processes; Wiley: New York, 1997. (7) Ehrenfeld, J.; Gertler, N. Industrial Ecology in Practice: The Evolution of Interdependence at Kalundborg. J. Ind. Ecol. 1997, 1 (1), 67.

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Received for review October 3, 2003 Revised manuscript received February 2, 2004 Accepted February 20, 2004 IE030746G